Performed semiannual maintenance on two gas furnace heating systems. Inspected all Performance, Mechanical and Safety systems. I did note while onsite that the inlet fuel pressure to both furnaces was under the minimum requirements. I raised the inlet pressure up and was able to properly adjust the gas manifold pressure of the furnaces to 11inWC to ensure 100% heating capacity. I also changed out both AprilAire filters (213 & 413) at the furnaces, as the main house area furnace filter was becoming dark in color and dirty which was beginning to cause an airflow restriction. Ensured both furnaces were powered back on cycled power prior to leaving. Everything is working at this time.